Ultra-wideband is a radio technology that transmits digital data across a very wide frequency range (3.1 to 10.6 GHz is currently approved by the Federal Communications Commission (FCC) in the United States). It makes use of ultra low transmission power, typically less than −41 dBm/MHz, so that the technology can literally hide under other transmission frequencies such as existing Wi-Fi, GSM and Bluetooth. This means that ultra-wideband can co-exist with other radio frequency technologies. However, this has the constraint of limiting communication to distances of typically 5 to 20 metres.
In one approach, ultra-wideband uses very short impulses, often of the duration of nanoseconds (ns) or less, to transfer information. These pulses give rise to spectral components covering a very wide bandwidth in the frequency spectrum, hence the term ultra-wideband, whereby the bandwidth occupies more than 20 percent of the centre frequency, typically at least 500 MHz.
In an alternative approach, the wide bandwidth is used to transmit information via a large number of orthogonal frequency carriers, organised into sub-bands; this is called Multi-Band Orthogonal Frequency Division Multiplexing (MB-OFDM).
These properties of ultra-wideband, coupled with the very wide bandwidth, mean that UWB is an ideal technology for providing high-speed wireless communication in the home or office environment, whereby the communicating devices are within a range of 20 m of one another.
FIG. 1 shows the arrangement of frequency bands in a Multi Band Orthogonal Frequency Division Multiplexing (MB-OFDM) system for ultra-wideband communication. The MB-OFDM system comprises fourteen sub-bands of 528 MHz each, and uses frequency hopping every 312 ns between sub-bands as an access method. Within each sub-band QPSK coding is employed to transmit data. It is noted that the sub-band around 5 GHz, currently 5.1-5.8 GHz, is left blank to avoid interference with existing narrowband systems, for example 802.11a WLAN systems, security agency communication systems, or those used in the aviation industry.
The fourteen sub-bands are organised into five band groups, four having three 528 MHz sub-bands, and one band group having two 528 MHz sub-bands, As shown in FIG. 1, the first band group comprises sub-band 1, sub-band 2 and sub-band 3. An example UWB system will employ frequency hopping between sub-bands of a band group, such that a first data symbol is transmitted in a first 312.5 ns duration time interval in a first frequency sub-band of a band group, a second data symbol is transmitted in a second 312.5 ns duration time interval in a second frequency sub-band of a band group, and a third data symbol is transmitted in a third 312.5 ns duration time interval in a third frequency sub-band of the band group. Therefore, during each time interval a data symbol is transmitted in a respective sub-band having a bandwidth of 528 MHz, for example sub-band 2 having a 528 MHz baseband signal centred at 3960 MHz.
The technical properties of ultra-wideband mean that it is being deployed for applications in the field of data communications. For example, a wide variety of applications exist that focus on cable replacement in the following environments:                communication between PCs and peripherals, i.e. external devices such as hard disc drives, CD writers, printers, scanner, etc.        home entertainment, such as televisions and devices that connect by wireless means, wireless speakers, etc.        communication between handheld devices and PCs, for example mobile phones and PDAs, digital cameras and MP3 players, etc.        
The antenna arrangements used in ultra-wideband systems are usually omni-directional, meaning that radio signals are emitted in all directions from an active radiating element, or elements. However, in future systems, which are targeted at very high data rate applications, there are benefits in using a number of higher gain elements, each of which covers a specific angular sector.
Although travelling wave elements can be used which offer the wide bandwidth required by an ultra-wideband network, an array of such elements is relatively large.
An array of low gain active elements (such as monopoles) can be used with a plane reflector surface which directs the signals in the required sectors. However, although the use of plane reflector surfaces to reflect radio signals in a desired direction is known, they are not suitable for use in ultra-wideband networks due to the large bandwidths used in these networks, because the physical distance between an active element and the reflective surface is normally chosen to be an optimum fraction of the operating wavelength. Due to the wide range of frequencies employed in a UWB system, an antenna with a fixed distance between the active element and the reflective surface will not function properly over the full bandwidth.
It is known that it is possible to adapt an antenna such that the physical distance between the active element and the reflector is changed according to the frequency being transmitted, for example by physically moving the reflector in relation to the active element, or vice versa. However, such arrangements are unsuitable for use with UWB systems in which the antenna must be capable of changing frequencies at extremely high speeds.
It is therefore an object of the invention to provide an antenna arrangement for use in an ultra-wideband system that overcomes the problems with the above conventional arrangements.